The field of this invention relates to a economical, safe, on-the-shelf decarboxylation process using mono silver salts for arylating and dimerizing aromatic compounds, and for coupling polysubstituted cyclic hydrocarbons and heterocyclics. The process also can be used to trifluoromethylate aromatic compounds.
Usually a decarboxylation reaction results in the generation of carbon dioxide and the concurrent replacement with hydrogen on the molecule. For example, the decarboxylation of benzoic acid yields benzene and carbon dioxide. Pyridine carboxylic acid goes to pyridine and carbon dioxide without any linkage occurring between two pyridine radicals. The presence of other carboxylic acid salts such as a sodium carboxylate of an aromatic acid will merely aid in the decarboxylation reaction at best and char to yield an ill-defined residue. The decarboxylation reaction products of this invention, on the contrary, are silver, carbon dioxide, and the coupled organic radicals of the silver carboxylates in the form of dimers, trimers, and multiples thereof, as well as coupled organic radicals of the other reactants present in multiple combinations. Surprisingly, mono silver salts of aromatic carboxylic acids form dimers, trimers, etc, when energy is applied instead of polymerizing to high polymers as do the di- and poly silver salts of polycarboxylic acids, as disclosed in copending U.S. application, Ser. No. 519,640 of Fields, Zimmerschied and Palmer.
In general, arylation or the coupling of aryl components by the formation of a bond between two aromatic carbons, either of aromatic or heteroaromatic compounds, is not by means of a decarboxylation reaction. The creation of such a bond, which almost invariably eliminates a hydrogen atom and has been defined as essentially a substitution reaction, is usually a homolytic aromatic substitution reaction. Among the reactions employed to achieve a desired homolytic substitution have been reactions involving diazo-, azo-, and related compounds, reactions involving peroxides and other sources of aroyloxy-radicals, photochemical reactions, and miscellaneous reactions such as use of certain Grignard reagents with specific reactants.
Many coupling reactions involving diazo-, azo-, and related compounds have been reported but frequently the reactions are specific to the preparation of certain compounds or these compounds decompose with explosive force unless precautions are taken. A biaryl can be formed in yields of 5 to 40% based upon the amine using an aqueous solution of a sodium azide with a neutral aromatic liquid stirred in the cold. Other examples are the use of a diazonium salt in aqueous acetone in the presence of cupric chloride to prepare biaryl derivatives, the preparation of nitrobiphenyls from a diazotised nitroaniline and benzene with aqueous sodium acetate or aqueous sodium hydroxide, the preparation of biphenyl when aniline is boiled under reflux with butyl and pentyl nitrite and benzene, and in the use of diazonium tetrafluoroborates in the presence of pyridine to form aryl radicals. A general method for homolytic arylation in a homogeneous medium is in the reactions of acylarylnitrosamines, which is exemplified by the decomposition of nitrosoacetanilide in benzene to give the biphenyl. Analogous reactions are possible with toluene, chlorobenzene, benzaldehyde and nitrobenzene. Homolysis of a diazophosphate to give aryl radicals which then can react with an aromatic solvent is another example.
Coupling reactions involving peroxides and other sources of aroyloxy radicals to arylate aromatic compounds are also well-known. But again these reactants have been reported frequently as decomposing with explosive force unless precautions are taken, or the reactions are specific to the preparation of certain compounds. Diaroyl peroxides react in aromatic solvents to yield the aryl radical, and result in the arylation of the aromatic solvent. Lead tetrabenzoate decomposes in aromatic solvents to give biaryls but because of the relative weight of lead, the process is highly uneconomic as compared with the disclosed invention. Phenyl iodosobenzoate upon decomposition gives the phenyl radical for the phenylation of the aromatic solvent.
Photochemical reactions are known to be sources of aryl radicals, in particularly the photolysis of organo-metallic compounds and aryl halides. p-Terphenyl, as an example, can be obtained from photolysis of 4-iodobiphenyl and benzene. 2,4,6-Tri-iodophenol plus benzene is known to give 2,4,6-triphenylphenol, for another example. Photochemical reactions however, often suffer from the handicaps of being specific to the preparation of certain compounds and whether a halide or an organometallic compound is available as a reactant. Further, the UV light that brings about the reaction causes additional reactions of the primary products.
Reactions for introducing fluorine into aromatic compounds are also well-known, such as the often-used Swarts reaction which utilizes antimony trifluoride. A chlorine atom in the molecule is replaced by a fluorine atom. The process is hence a two-step process, requiring first the introduction of chlorine atoms. The Swarts reaction can also involve problems of control, as many aromatic chlorides react very rapidly with antimony fluoride.